[sdiy] Sallen-Key Bessel Low Pass - behaviour and RC values

Scott Gravenhorst music.maker at gte.net
Thu Feb 6 15:03:51 CET 2014


Justin Owen <juzowen at gmail.com> wrote:
>Hello,
>
>I've been asked to help set up an anti-alias (input) and 
>reconstruction (output) filter for a micro-based project a friend 
>is working on. I'll be using a Bessel type Sallen-Key Low Pass 
>config for both - but I had some initial questions. 
>
>In a general sense, my understanding is that the real world 
>differences in behaviour/performance between Bessel and 
>Butterworth in a SallenKey config are mainly down to Q and 
>cutoff? 
>
>...and the differences in component selection is that for a 
>Butterworth topology there is a 2:1 ratio between the values of 
>C1 and C2 (or C2 and C ATCMB...) and for a Bessel C1 = C2. 
>
>Is that right?
>
>Secondly, there seems to be two schools of thought on whether to 
>set the filter cutoff frequency at Nyquist (which I believe is 
>currently 96KHz) or around the highest frequency the DAC will 
>output (in our case I believe this is about 16KHz). 

I will assume this is an audio project.  As such, I don't believe we need much
more bandwidth than that of human hearing.  As such, a cut off of 96 kHz seems a
bit extreme.  I would put cutoff somewhere between 16 kHz and 20 kHz.

>Any thoughts on this from those who've been round this block before?
>
>Lastly - I hope you guys don't me asking this on list but I'd be 
>curious to hear from Tom W. and Olivier G. about their RC 
>selections of 12K/10nF (plus some gain?) and 39K/100pF - and 
>anyone else on there fave or suggested RC values. 
>
>Thanks!
>
>- J
>
> 
>
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-- ScottG
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